The present invention is concerned with the production of ammonia from a hydrocarbonaceous feed, including the preparation of an ammonia synthesis gas containing hydrogen, more particularly, with the preparation of a synthesis gas comprising hydrogen and nitrogen in selected molar ratios and the reacting of these constituents to form ammonia.
As used herein and in the claims, the term "hydrocarbonaceous feed" is intended to include, without limitation, hydrocarbon feeds of all types, as well as alcohols such as ethanol, methanol and mixtures thereof, and biomass-derived feeds which normally contain compounds of carbon, hydrogen and oxygen, and sometimes compounds of nitrogen and sulfur, as well as one or more of the foregoing elements in elemental form. Such biomass-derived feeds may be obtained by any suitable process such as fermentation of grains or other materials including food products generally, from treatment of agricultural by-products and waste products, or by distillation or combustion (with insufficient oxygen for stoichiometric reaction) of wood and/or other cellulosic products and by-products.
Reference hereinbelow and in the claims to a "synthesis gas" or "synthesis gases" will be understood to mean a gas mixture comprising hydrogen and nitrogen, possible with other constituents, such as H.sub.2 O, carbon dioxide, sulfur compounds, inert gases and the like. Such other constituents are often removed prior to or during the ammonia synthesis operation in which the synthesis gas is reacted.
Ammonia synthesis gas may be prepared by the partial oxidation of hydrocarbonaceous feeds, such as the heavier hydrocarbons, e.g., fuel oil and coal, and by steam reforming of hydrocarbonaceous feeds, such as the lighter hydrocarbons, e.g., natural gas and naphthas. Processes to derive synthetic gases from methanol or coal derived hydrocarbons are also known.
Generally, difficulties associated with the preparation of synthesis gases from heavier feedstocks favor the use of light naphthas or natural gas when a hydrocarbon is the source of the hydrocarbonaceous feed. However, shortages of such light hydrocarbon feeds indicate the need for an economical process for generating a synthesis gas from heavier hydrocarbon feedstocks, such as normally liquid hydrocarbons.
Steam reforming is a well known method for generating synthesis gas from light hydrocarbon feeds and is carried out by supplying heat to a mixture of steam and a hydrocarbon feed while contacting the mixture with a suitable catalyst, usually nickel. However, steam reforming is generally limited to paraffinic naphtha and lighter feeds which have been de-sulferized and treated to remove nitrogen compounds, because of difficulties in attempting to steam reform heavier hydrocarbons and the poisoning of steam reforming catalysts by sulfur and nitrogen compounds. Further, steam reforming for ammonia production generally must be carried out in two stages, a primary steam reforming stage and a secondary steam reforming stage. The former requires a tubular furnace containing catalysts disposed within the tubes and means to supply heat to the tubes in order to sustain the endothermic steam reforming reaction. Secondary reforming is usually carried out in a separate vessel to which oxygen is added to carry out a partial combustion to supply additional heat for the endothermic steam reforming step. Such processes are well known to the art, as indicated by the article "Checklist for High Pressure Reforming" by Orlando J. Quartulli appearing at pages 151- 162 of Hydrocarbon Processing, April, 1965, Vol. 44, No. 4.
Another known method of obtaining hydrogen from a hydrocarbon feed is the partial oxidation process in which the feed is introduced into an oxidation zone maintained in a fuel rich mode so that only a portion of the feed is oxidized. Steam may be injected into the partial oxidation reactor vessel to react with the feed and with products of the partial oxidation reaction. The process is not catalytic and requires high temperatures to carry the reactions to completion, resulting in a relatively high oxygen consumption. On the other hand, the partial oxidation process has the advantage that it is able to readily handle hydrocarbon liquids heavier than paraffinic naphthas and can even utilize coal as the source of the hydrocarbon feed.
Catalytic autothermal reforming of hydrocarbon liquids is also known in the art, as evidenced by a paper Catalytic Autothermal Reforming of Hydrocarbon Liquids by Maria Flytzani-Stephanopoulos and Gerald E. Voecks, presented at the American Institute of Chemical Engineers' 90th National Meeting, Houston, Tex., April 5-9, 1981. Autothermal reforming is defined therein as the utilization of catalytic partial oxidation in the presence of added steam, which is said to increase the hydrogen yield because of simultaneous (with catalytic partial oxidation) steam reforming being attained. Steam, air and a No. 2 fuel oil are injected through three different nickel particulate catalysts and the resulting product gases contained nitrogen, hydrogen and carbon oxides.
In Brennstoff-Chemic 46, No. 4, p. 23 (1965), a German publication, Von P. Schmulder describes a Badische Anilin and Soda Fabrik (BASF) process for autothermal reforming of gasoline utilizing a first, pelletized, platinum catalyst zone followed by a second, pelletized nickel catalyst zone. A portion of the product gas is recycled to the process.
Disclosure of the utilization of a noble metal catalyzed monolith to carry out a catalytic partial oxidation to convert more than half of the hydrocarbon feedstock upstream of a stream reforming zone is discussed in abstract entitled "Evaluation of Steam Reforming Catalyst for use in the Autothermal Reforming of Hydrocarbon Feed Stocks" by R.M. Yarrington, I.R. Feins, and H.S. Hwang (National Fuel Cell Seminar, July 14-16, 1980, San Diego, Calif.) The abstract noted the unique ability of rhodium to steam reform light olefins with little coke formation and noted that results were obtained for a series of platinum-rhodium catalysts with various ratios of platinum to total metal in which the total metal content was held constant.
U.S. Pat. No. 4,054,407, assigned to the assignee of this application, discloses two-stage catalytic oxidation using platinum group metal catalytic components dispersed on a monolithic body. At least the stoichiometric amount of air is supplied over the two stages and steam is not employed.
U.S. Pat. No. 3,481,722, assigned to the assignee of this application, discloses a two-stage process for steam reforming normally liquid hydrocarbons using a platinum group metal catalyst in the first stage. Steam and hydrogen, the latter of which may be obtained by partially cracking the hydrocarbon feed, are combined with the feed to the process.
The present invention provides a highly efficient method for producing synthesis gases from hydrocarbonaceous feeds, including hydrocarbons, which attains excellent yields in a relatively compact and simple apparatus.